The invention relates to a method of determining the profile of a surface using contactless distance measurement, and to an apparatus for also determining the profile of a surface using contactless distance measurement, wherein a light pencil which has been scattered and reflected from the surface is split into two light pencils having substantially the same distribution of intensities in a plane transverse to the direction of propagation.
A method and an apparatus for determining the profile of a surface using contactless distance measurement are known from Ger. OS 34 35 033. According to that document the method of measuring heights and distances in the plane of an object may be carried out without having to focus the imaging optics if the arrangement follows the Scheimpflug rule. In this way, most importantly, the accuracy of the determination can be improved by linearly moving a light spot on the photodetector. The light spot which is the smallest which can be maintained in place is accordingly produced with an edge focused onto the surface to be measured and from there, is imaged on the photodetector. When a laser is used as the light source, a rough surface causes a high level of noise in the measured value, particularly in the form of numerous so-called "speckles" in the light spot being imaged on the photodetector. The resulting noise in the photodetector signal is greater when the light spot on the surface which is to be measured is made smaller. For roughness measurements, the diameter of the spot is a few to several microns; and for this reason a laser light should be used. However, the ability to form the image of a sharp edge is limited because of the smallness of the light spot.
It is known from Ger. Pat. 2,945,251 that changes in distance, and thereby surface profiles, can be measured very precisely using a triangulation method. For this purpose, a laser beam is directed obliquely on the surface to be measured. With the aid of optics oriented perpendicularly with respect to the surface, the light spot is imaged onto a position-sensitive photodetector. The light spot imaged onto the photodetector shifts proportionally to the change in distance of the sample. With the aid of a differential photodiode or a row of photodiodes acting as the photodetector means, the shift in the light spot and thereby the change in the distance from the laser to the surface to be measured is determined, e.g. by electrical means. The principal shortcoming of this method is that when a rough surface is measured using laser light, the light spot is broken up into numerous "speckles", leading to appreciable noise in the measured values.
Sensitive measurement systems have a small scope of measurement. It is therefore a general problem to bring the measuring surface into the existing measuring zone. This problem is, for instance, solved in Morita, U.S. Pat. No. 4,732,485, by using a supplemental measurement system for bringing the measurement surface into a correct position.
Another problem with the prior art is the effect of reflections on the measurement system. Since the diodes will measure any kind of intensity alteration, certain reflections from the measuring surface occurring, when not a surface roughness, but a depression or elevation, completely outside of the scope of measurement cannot be distinguished and will be seen as measured values.